In an injection molding machine which comprises a mold half mounted to a movable platen and having a core plate, and another mold half mounted to a stationary platen and having a cavity plate, the mold clamping or locking force is produced as the movable platen is moved along tie bars over a small distance from a mold touch position in which the core plate of one mold half closely contacts the cavity plate of the other without leaving any clearance, toward the stationary platen to a lockup position. The mold clamping force, i.e., the compressive force acting on the mold halves when they are locked up, is equivalent to the internal stress of the tie bars, which are made of an elastic material, produced when the movable platen is held in the lockup position. At this time, the tie bars expand slightly. The mold clamping force is therefore determined definitely by the expansion of the tie bars produced at the time of the lockup. To adjust the mold clamping force in practice, the expansion of the tie bars is adjusted by varying the small amount of movement which the movable platen makes between the mold touch position and the lockup position, i.e., the drive amount by which the movable platen is driven.
Therefore, in setting the mold clamping force during a molding operation, only data on the drive amount and the mold touch position where the drive of the movable platen is started are required. Generally, the mold touch position is detected during mold thickness adjustment, with a mold mounted on the injection molding machine, and the mold touch position and the drive amount are set in the control section of the injection molding machine, so that the mold clamping operation is performed in accordance with the set values.
However, the mold touch position, which is determined in accordance with the mold thickness, varies with lapse of time depending on the expansion and contraction of the mold caused by temperature changes, while the actually used mold touch position and the drive amount remain constant as set during the mold thickness adjustment. Therefore, if the temperature of the mold decreases after the mold thickness adjustment, the mold clamping force actually obtained will be insufficient, whereas if the mold temperature increases after the mold thickness adjustment, the mold clamping force obtained will be greater than necessary, thus failing to keep the mold clamping force constant.
A countermeasure conventionally employed to eliminate such disadvantage is to perform the mold thickness adjustment for detecting the mold touch position after the temperature of the mold has reached a thermal equilibrium in which the mold thickness is stable. Therefore, even in the case of using a resin with which molding conditions are never influenced by changes of the mold temperature, molding operation cannot be started immediately after the mold is mounted on the injection molding machine.
Even after the mold temperature has reached a thermal equilibrium, it is still difficult to keep the mold temperature constant all the time since it can be influenced by changes in ambient temperature. For example, while unmanned operation of the injection molding machine is continued for a long period of time, the mold temperature may be changed due to the influence of the ambient temperature so that the mold clamping force varies correspondingly, thus possibly causing defective molding.